Full color information display systems for display of high definition video information and complex pictorial and graphics images must be able to provide high image resolution for precise edge definition and image sharpness; high display and image luminance for maximum image brightness in a variety of display environments; and precise, predictable control over color synthesis and reproduction using the largest color gamut available. For purposes of the discussion herein, the term "full color" display means a display which is capable of producing color from the full spectrum of visible light, and which uses at least three additive or subtractive primary colors to produce the full spectrum.
Direct view, full color visual displays predominantly use an additive color system based on spatial juxtaposition, or spatial proximity, for the production of full color images. A single, full color picture element, or "pixel", of a displayed image is produced by the spatial integration of very small, juxtaposed primary (red, green, and blue) color sub-picture elements, or subpixels. "Pixel" and "image pixel" are defined herein as the smallest information element in a displayed image. The resolution of an image is determined by its pixel density. "Subpixel" and "image subpixel" are defined herein to mean a single primary color element that is used, along with two other primary color elements, to form a color from the full spectrum in an image pixel. Additive spatial proximity color synthesis requires high subpixel density (resolution) because the projected angular subtense of the primary color elements must be encompassed within the spatial integration zones of the human visual system in order for the eye to integrate a set of individual primary color subpixels into the single mixture color to be formed by the image pixel.
A leading display technology for high information content, full color visual displays is backlit liquid crystal display (LCD) technology, and, in particular, active-matrix addressed color liquid crystal display (AMCLCD) technology. Typically, full color liquid crystal displays utilize a matrix of individually addressable liquid crystal light valves (LCLV) with integral red, green, and blue color filters for forming a full color image using the technique of additive spatial juxtaposition.
From the perspective of the display's hardware, each primary color subpixel of an image pixel must be individually controllable for luminance, generally along some range from a minimum of no light to the maximum light the subpixel is capable of producing. Thus, a primary color image subpixel is the equivalent of, and will be referred to herein as a "display pixel". The hardware requirement in additive spatial proximity color synthesis for three "populations" of spatially separated primary color display pixels results in a reduction of available image sampling resolution for a display device of a given size. For the display of large full color images in particular, additive spatial juxtaposition color synthesis alone is generally not an efficient method for generating full color images because of the excessive cost associated with increasing the size of the display in order to achieve high image resolution. In addition, significant losses in display luminance and perceived brightness result from the fact that each of the three primary colors, without regard to its individual contribution to overall luminance or perceived brightness, generally occupies an equal amount of the available, active light emitting surface area of the display.
Another method for generating full color images is based on additive spatial superposition in which a full color image is produced by the spatial registration of separate images, each comprised of typically one primary color, and optically fused into one full color image for viewing by the observer. Such a system is the predominant method used in color projection displays. Typically, three images corresponding to red, green, and blue primary colors are generated, requiring three separate imaging (optical) paths. Because each display pixel is equivalent to an image pixel and is capable of full color and luminance control, and because each of the color images is generated at full spatial resolution, the additive spatial superposition method of color synthesis achieves excellent image resolution and can also achieve relatively high overall luminance and perceived brightness. For these reasons, spatial superposition of separate color images provides a very feasible color synthesis method for producing large full color display images, such as those required in high definition television or comparable visual information display systems.
However, multiple (three or more) optical path, full color, spatial superposition display systems require precisely controlled hardware and optical elements to achieve exact image registration and alignment in order to maintain color purity and image sharpness. In addition, color display systems using projected superimposed images tend to be large, complex, and costly as a result of the separate optical paths and sets of imaging elements needed.
The concurrently filed and commonly assigned U.S. patent application of Silverstein, et al., entitled "Two Path Liquid Crystal Light Valve Color Display" recognizes the deficiencies of each of the two methods of color production and proposes a full color display that takes advantage of psychophysical properties of the human visual system in order to make a full color display which is optically simpler to construct than a three optical path system, and which eliminates some of the critical image alignment problems in three optical path systems. Specifically, the full color display disclosed in Silverstein, et al. generates and displays a composite full color image in two optical paths by combining a red and green color image with a blue color image using additive spatial superposition of the images. In one embodiment, the red and green color image is formed by additive juxtaposition of red and green display pixels in one liquid crystal image forming means, and the blue color image is formed in a second liquid crystal image forming means.
The invention disclosed in Silverstein, et al. utilizes research findings that recognize certain factors about the human visual system. In particular, research has shown that the eye's peak spatial response to blue light occurs at approximately one half the spatial frequency of peak spatial response for red or green light and half again the spatial frequency for achromatic, or luminance, signals, indicating that blue light contributes only a minor amount to image resolution factors such as image shape and spatial detail. As a result, neither the resolution nor the alignment of blue image pixels in an image created by additive spatial superposition is critical to image quality since misalignment is not easily detected by the eye. Conversely, misalignment of red and green images is more easily detected by the eye, imposing a critical registration constraint on the red and green images in three optical path systems. It is also known that the photopic response of the human eye to blue light is low and inefficient, and thus, short wavelength (blue) light provides a much smaller contribution to overall perceived brightness than long (red) and medium (green) wavelength light of the visible spectrum. At the same time, the highest possible luminance contribution of blue light improves the display's maximum luminous output and overall color balance or white point, and achieves a larger and more balanced color gamut.
Silverstein et al. recognized that combining the red and green image forming paths into a single optical path eliminated the need for the critical alignment of those images in the final image formed by superposition. In addition, they recognized that increasing the display pixel size and reducing the resolution of the blue light image would result in an increase in the overall space-average intensity of the short wavelength light contribution, without increasing the intensity of the light source and without reducing the effective image resolution. The separate optical path for short (blue) wavelength light, and the formation of the blue image with a lower sampling density, or resolution, increases the overall perceived image and display brightness, provides a brighter display white point, and provides brighter color rendition for colors having a blue component.
However, once treatment of the blue light has been optimized in this manner, the overall perceived brightness of the full color image and the luminous efficiency of the two optical path display device is largely determined by the luminous efficiency of the optical path which handles the production of the red and green image. In general, when a display operates in transmissive mode, display and image luminance, and luminous efficiency are determined by both the luminance of a backlight, placed behind the display, and the transparency of the display. These factors, in turn, affect the color fidelity of the display. Thus, more efficient generation of the red and green radiation emanating from the backlight in the red and green optical path and more efficient control of the radiation through the red and green image forming light valve afford the viable opportunities for maximizing the overall perceived brightness and luminous efficiency of the entire display.
As described by Silverstein, et al., the red and green image in the two path display is produced in one embodiment using a mosaic of individual (narrow band) red and green color filters arranged on a single layer. As is generally known in the art, such a color filter mosaic is typically positioned immediately adjacent to the liquid crystal panel such that each individual color filter is in registration with a respective individual display pixel and transmits only the desired portion of broadband (white) light while absorbing all other wavelengths. In the case of the red and green optical path, the backlight is a yellow light source with associated collimating optical elements for controlling the spatial distribution of the light before it enters the red and green image forming light valve.
The absorptive red and green color filter mosaic, however, is a highly inefficient use of the energy contained in the yellow light source, since it absorbs a substantial amount of the incident light, as much or more than two-thirds, during red and green color selection, consequently reducing the luminous efficiency of the red and green optical path.
In another embodiment of the two path full color display disclosed by Silverstein et al., the red and green color filter layer is eliminated, and the red and green image is formed using a pair of aligned liquid crystal image forming sources with a pair of color selecting polarizers. Elimination of the color filter mosaic and use of the two aligned liquid crystal image forming sources and color selecting polarizers eliminates the disadvantages associated with the color filter layer, but introduces a requirement for maintaining the careful alignment of the light rays as they pass through both liquid crystal image forming sources. That is, in order to achieve good color fidelity and maintain high overall brightness in the red and green optical path, the yellow light must be carefully channeled to pass through both liquid crystal image forming sources so that substantially all the light that passes through each individual display pixel of the first liquid crystal image forming means also passes through the respectively aligned display pixel of the second liquid crystal image forming means and through the color selecting polarizers without distortion. Substantially no stray light must pass through adjacent pixels. A conventional optical element or set of optical elements is provided for substantially collimating the yellow light as it passes through both liquid crystal image forming sources. Those skilled in the art are aware that substantially collimating the light through the liquid crystal light valves has a tendency to reduce the amount of source light collected and available to the liquid crystal light valves for image formation, thus reducing the overall brightness.
Further complicating the requirement for carefully channeling the yellow light through respectively aligned display pixels is the structure of the display pixels themselves. The transparency of a display is normally reduced by the amount of inactive area in the active matrix addressing elements comprising the transparent conductive electrode, i.e., the presence of the opaque active elements and interconnect within the display pixel. Since the transparent conductive electrode may comprise an area from about thirty percent (30%) to about eighty percent (80%) of the area of each display pixel, depending on the display application, considerable display luminance may be lost by light which is blocked by the opaque addressing elements and which never enters the transparent portion of the pixel. Conventional light collimating techniques do not address the problem of light blocked by the opaque portion of the display pixel.